I have managed to isolate this down to the wp_head(); call in my header and THINK the jQuery version that WordPress loads, wp-includes/js/jquery/jquery.js?ver=1.2.6 is not compatible with the scripts I am using which used to load jquery-1.2.3.pack.js

I ran into this issue before and had to hack the plugin that was relying on an older version of jquery. It worked but it was only going to work on my site. I couldn’t expect anyone else to do the same thing. I just abandoned the use of the plugin that relied on a prev version of jquery.

All this hassle has made me realise that if I want the benefits of WordPress MU then I need to switch the ‘sliding’ components that were working on the old, standalone wordpress blogs to something that keeps up (well nearly at least!) with WPMU and it’s jQuery updates

I had the same problem with jcarousel and buddypress, and paid a few dollars over at wpquestions to get the fix, which involved editing a line of code in a buddypress js file which handles easing. The full answer is here if anyone else gets stuck: http://wpquestions.com/question/show/id/1615